Subtopic: Psychrometry – properties of air-water vapour mixture
Question Statement:
The percentage absolute humidity of air becomes equal to the percentage relative humidity, when
Options:
(A) absolute humidity of air is equal to relative humidity
(B) saturated humidity of air is equal to relative humidity
(C) air is almost or completely dry
(D) air is almost or completely saturated
Official Answer Key:
C, D
Step-by-Step Solution & Explanation
Percentage absolute humidity (percentage humidity) is defined as the ratio of actual humidity ratio to the saturation humidity ratio at the same temperature, ×100. Percentage relative humidity is the ratio of actual vapour pressure to saturation vapour pressure at the same temperature, ×100. These two percentages differ by a correction factor that depends on the ratio of (total pressure − saturation vapour pressure) to (total pressure − actual vapour pressure).
This correction factor approaches unity — making percentage humidity ≈ percentage RH — in TWO limiting cases: (i) as the air approaches saturation (RH→100%), the actual and saturation vapour pressures converge, shrinking the correction factor to 1 — this is statement (D); and (ii) as the air approaches completely dry conditions (both actual vapour pressure and humidity ratio →0), the correction factor also tends to 1 because the small partial-pressure terms become negligible relative to total pressure — this is statement (C). Statements (A) and (B) merely restate the condition being asked about (equal to itself, or equal to "saturated humidity", an undefined comparison) without identifying the actual physical condition. Answer: (C) and (D).
Section 7 — Dairy and Food Engineering
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